Acrolein and acrylic acid are produced in large scale for use as chemical building blocks mainly for producing polymers. The totally dominating process for production of acrolein and/or acrylic acid is today a gas-phase oxidation of propene, see for instance Ullmann's Encyclopedia of Industrial Chemistry, Release 2002, 6th edition. In view of the price difference between propene and propane and as propene is used as feedstock for producing many other products, it would be advantageous to have a competitive process, which uses propane instead of propene as raw material for preparing acrolein and acrylic acid.
EP patent application No. 117146 proposes preparing acrolein and/or acrylic acid from propane by a two-stage process, where propane is subjected to a partial oxydehydrogenation with molecular oxygen in the first stage to give propene. The product gas from the first stage is then used directly for the preparation of acrolein and acrylic acid by gas-phase catalytic propene oxidation in the second stage. A disadvantage of this and similar processes is that the reaction has to be carried out in two reactors operating at different reactions conditions.
Processes for converting propane into acrolein and/or acrylic acid in a single reactor are also known. U.S. Pat. No. 6,541,664 proposes a process, where two different catalysts are loaded spatially in succession into the same reactor. The first catalyst helps transforming propane into propene, which is then converted to acrolein and/or acrylic acid over the second catalyst.
Similar processes where the trouble of producing and loading two different catalysts is avoided have also been suggested. U.S. Pat. No. 5,380,933 describes catalysts containing Mo, V and Te oxide that are able to accomplish this, and a number of patents describe subsequent modifications of this catalyst system, see for instance U.S. Pat. Nos. 6,160,162, 6,504,053, 6,624,111 and 6,700,015. This catalyst system has the disadvantage that the catalyst contains Te and this component might be considered undesirable because it is known to be toxic and volatile. Even though the ecological problem can be handled the long time stability of these catalysts will be limited.